Otosclerosis is a common cause of progressive conductive hearing loss in which softening and hardening of minute areas of the ossicles (malleus, incus and stapes) in the middle ear produce abnormal bone growth and impede conduction of sound vibration from the eardrum to the inner ear. In about ten percent of patients with otosclerosis, the bone growth spreads to the stapes bone in particular, the final link in the middle ear chain. The stapes is a small stirrups-shaped bone with its base resting in a small groove, commonly called the oval window, in intimate contact with the inner ear fluids. When the amount of otosclerosis at this location is significant, as determined by careful hearing tests, a stapedectomy (or stapedotomy) is the treatment of choice.
A stapedectomy is a microsurgical technique in which all or a portion of the stapes is replaced by a prosthesis. When the procedure was first introduced in the 1950s, many surgeons became skilled in the technique and while it is now performed by many more surgeons, on an average, there are relatively few performed per surgeon. Hence, proficiency is harder to maintain for the occasional stapes surgeon.
A stapedectomy is usually performed through an incision in the ear canal under local or general anesthesia. A flap consisting of canal skin and the tympanic membrane (eardrum) is elevated and the posterior superior bony external auditory canal is drilled away to expose the stapes, incus, and chorda tympani (facial nerve). The ossicles are palpated to confirm fixation of the stapes and mobility of the malleus and incus.
With care taken to preserve the chorda tympani, the joint between the incus and the stapes is separated with a knife, and a laser or other microsurgical instrument severs the stapes tendon and one crus (leg) of the stapes. The arch of the stapes bone may then be removed by fracturing off the other crus allowing the footplate to remain in the oval window. A laser is next employed to form a minuscule hole in the footplate for posting the stapedial prosthesis. In some cases, the footplate is also removed by a so-called "large hole" technique and a vein grafted to the internal wall of the tympanum to cover the opening and to support the prosthesis.
After a hole is made in the footplate (or tissue is placed over the opening to the inner ear made after removing the footplate) one end of a biocompatible plastic or metal piston-like stapedial prosthesis of proper length is posted in the hole and the other end attached to the incus. A piece of fat or other tissue is taken, such as from a small incision behind the ear lobe, to seal any hole in the window, and the eardrum is folded back into its normal position with a small gelatin sponge to hold it in position.
A critical part of the procedure is attaching the prosthesis around the lenticular process of the incus due to its minuteness and delicate nature, typically about 3.5 mm to 6 mm long and 0.6 mm to 0.8 mm diameter, For instance, in U.S. Pat. No. 5,370,689 to Causse one end of the prosthesis fabricated of PTFE is posted in a hole drilled in the exposed footplate and a split eyelet at the other end must be crimped around the incus. In U.S. Pat. No. 3,714,869 to Shay Jr. one end of the prosthesis is placed on a vein graft invaginated into the oval window, and a split eyelet at the other end must be forced open by elastic deformation to fit onto the incus. Elastic recovery capacity of the eyelet causes it to restore to its original form in about 20 minutes and grip the incus firmly. U.S. Pat. No. 3,838,468 to Armstrong discloses a stapedial prosthesis for use in cases where the footplate is also removed. A piston is fixed at one end to a vein graft for covering the oval window. A wire of stainless steel, platinum, gold or like biocompatible material shaped like a shepherd's crook extending from the other end, is crimped about the lenticular process of the incus. U.S. Pat. No. 5,433,749 to Clifford et al. discloses a stapedial prosthesis of metal or plastic in which one end of a piston extends into the fluid in the inner ear and the other end is secured to the incus by a separate heat-shrinkable sleeve when heat is applied as by a laser.
It is readily apparent that great care and skill are required to secure these and similar prostheses to the lenticular process of the incus. The minute size of the prostheses also makes them extremely difficult to manipulate into proper position for tightening around the incus, even with state-of-the-art microsurgical instrumentation. Once in place, if the prosthesis is not tightened sufficiently about the incus, fluctuating hearing loss, dizziness, or extrusion of the prosthesis may occur. If it is too tight, necrosis of the incus may occur. In either case, the tightening procedure in itself may cause trauma to the delicate middle ear structures, including fracture or subluxation (dislocation) of the incus.
Other otologic protheses may be implanted by similar procedures directly between the malleus and the footplate of the stapes or the oval window of the inner ear.